Italian patent application TO2014A001045 discloses a rolling bearing unit comprising:                a radially outer race which is stationary during use and has a central and circular groove on an inner surface thereof;        two radially inner races which are rotatable during use;        two rings of conical rollers arranged between the radially outer race and the two radially inner races; and        a grease metering device housed inside the groove of the radially outer race and comprising, in turn, a radially outer ring on which at least one radially inner projection defined by a continuous vertical wall is engaged.        
In the conical roller bearing unit of the type described above, the grease is introduced into the central zone situated between the two rings of conical rollers and, typically, types of oil-based soap-thickened grease, which has a good thermal, oxidation and mechanical stability as well as excellent water-resistance and anti-rust properties, are used. Therefore, the continuous vertical wall of the grease metering device has the function of creating a kind of obstacle to the free outflow of the grease, preventing possible migration of the grease from one ring to the other ring of conical rollers, in particular during transportation of the bearing unit, and allowing suitably controlled dispensing of the grease during operation of the bearing unit. However, with the grease metering devices of the known type, during the operating conditions of the bearing, it has been noted that the grease migrates between the various parts of the bearing unit in an entirely random manner, owing to various factors, most of which cannot be controlled, with the result that often some zones requiring more lubrication receive small amounts of grease and are subject to premature wear, while other zones requiring less lubrication receive too much grease, resulting in undesirable accumulations with a consequent increase in the leakages due to friction.